Plasma deposition of thin organic films has increased in importance in a number of areas of technology in recent years. Overlayer films on solid substrates can be produced without exposing the solids to solvents or high energy radiation. Films deposited by this method show many desirable characteristics including ease of preparation, uniformity, conformal coverage of complex substrates, excellent adhesion to a variety of substrates and the ability to generate unique chemistries. In addition, the overlayer films do not penetrate significantly into the substrate and, therefore, do not affect the mechanical properties of the substrate. These films are typically free of leachable components and can be designed to prevent leachable components in the substrate from diffusing out.
The primary disadvantage of plasma deposited films is that they are of ill-defined chemistry. Because of the complexity of the composition of the plasma phase and the many possible reaction mechanisms that may lead to the incorporation of a particular atom or functional group into the growing film, the final film obtained from these types of depositions is hard to predict or control. Participating reactions have been grouped into plasma phase and surface reactions, and generalized mechanistic schemes for these reactions have been developed. H. Yasuda, J. Polym. Sci. Macromol. Rev., 16, 199-293 (1981). During a plasma deposition, an organic compound (precursor), which may or may not be polymerizable by traditional methods, is dissociated and rearranged to produce a new structure and finally deposited onto a substrate in a chemical environment that is typically quite different from the original. For example, if an organic amine is introduced into the plasma environment with the intent of introducing amine functionalities on the film surface, a wide range of nitrogen containing organic functional groups will actually be obtained. Consequently, chemical tailoring of films by this method has often been quite laborious and typically phenomenological in approach.
Bell and coworkers were among the first groups to study the effects of deposition variables on film chemistry. K. Nakajima, A. T. Bell, and M. Shen, J. Apl. Polym. Sci., 23, 2627-2637 (1979). They produced films under conditions where the dominant functional group is --CF.sub.2 --. They noted that these conditions also favored low deposition rates, and that at conditions which produce higher deposition rates, the number of --CF.sub.2 -- groups decreases and a more crosslinked polymer is formed. However, they did not disclose or suggest that a temperature differential between substrate and deposition chamber could be employed to control chemistry of the deposited film.
The dependence of film chemistry on operational variables has thus far been mainly expressed in terms of the degree of precursor fragmentation that occurs between the time of exposure to the plasma and the incorporation into the film matrix. Thus, if a functional group of interest is to be incorporated into a film without fragmentation or rearrangement, the operational parameters of the deposition must be identified which affect fragmentation and these must be tailored to prevent extensive fragmentation.
Clark and coworkers also studied the effects of deposition variables on film chemistry. D. T. Clark, Pure and Appl. Chem., 54, 415-438 (1982); D. T. Clark and M. Z. Abrahman, J. Polym. Sci., Polym. Chem. Ed., 20, 691-706 (1982); and D. T. Clark and M. Z. Abrahman, J. Poly. Sci., Polym. Chem. Ed., 20 1729-1744 (1982). Using X-ray photoelectron spectroscopy (XPS) analysis of various types of fluorocarbon deposits, they were able to show that the films deposited at low power levels incorporated precursor with a lower degree of functional group (e.g., aromatic) fragmentation than at high power levels. They also showed that higher system pressure during the deposition favors less fragmentation. By comparison with deposition rate data, they surmised that the W/FM parameter, which determines deposition rate characteristics, may be an indicator of the degree of fragmentation the precursor undergoes during deposition. Here, W is the RF power supplied to the plasma, F is the precursor flow rate, and M is the precursor molecular weight.
The W/FM parameter, which was first introduced by Yasuda et al., is a measure of the energy supplied per unit mass of the precursor introduced to the plasma. H. Yasuda and T. Hirotsu, J. Polym. Sci., Polym. Chem. Ed., 16, 743-759 (1978). It is therefore reasonable that the amount of precursor fragmentation which occurs before it is finally deposited into the film matrix is related to W/FM. The effect of W/FM on film chemistry was also investigated by Inagaki et al. N. Inagaki, M. Doyama, and H. Igaki, J. Polym. Sci., Polym. Chem. Ed., 22, 2083-2093 (1984). They too, confirmed the concept that low W/FM conditions were conducive to precursor substituent effects. The effects of methyl, methoxy, and vinyl substituents were studied using XPS elemental analysis and infrared adsorption spectroscopy.
An alternative, but similar, formalism was provided by Evans and Prohaska. J. F. Evans and G. W. Prohaska, Thin Solid Films, 118, 171-180 (1984). They calculated a power to molecule ratio by the expression:
P/M=RTW/PVN.sub. A
where T is the ambient temperature, P is the steady state pressure of the discharge, V is the reactor volume, and W is the net stead state power delivered to the reactor. For two different precursor systems, they found deposition rate was increased at higher P/M levels and that chemical functionalities of the precursors are retained and incorporated into the deposited films at low P/M conditions.
The effect of several other system variables has also been discussed as a means of varying the deposited film chemistry or as a way of obtaining films of more predictable chemistry by reducing the fragmentation of the precursor. One example of this is the use of pulsed radio frequency (RF) discharges to initiate plasma-induced polymerization during the powered portion of the pulse period which can continue during the unpowered portion. Such a scheme would tend to incorporate precursor in a relatively unfragmented state during the unpowered portion of the deposition. H. Yasuda and T. Hsu, J. Polym. Sci., Polym. Chem. Ed., 15, 81-97 (1977). Chemical properties of films were found, by Yasuda and Hsu, to vary significantly when pulsed discharges were used. The degree of change produced by this manner was found to depend on the precursor structure. In particular, electron spin resonance (ESR) signals of free spins from the deposited films and the glass substrates were found to vary substantially for some precursors. Nakajima, et al., performed similar investigations using one of the precursors investigated by Yasuda and Hsu, namely tetrafluoroethylene. K. Nakajima, A. T. Bell, and M. Shen, J. Appl. Polym. Sci., 23, 2627-2637 (1979). They found essentially no difference in deposition rate and XPS analysis resulting from the pulsed discharge mode.
Another method that has been investigated for incorporating precursor in a relatively unfragmented state is the placement of substrates in a position in the reactor which is adjacent to the plasma but not completely submersed in it. The rationale for doing this is that outside the bulk of the plasma the proportion of unfragmented precursor molecules will be greater and these can be incorporated into the film by propagation reactions, while initiation and termination reactions would be less frequent. Moreover, the degree of ion, electron, radical and photon bombardment will depend on the location of the substrate with respect to the plasma and the resulting film chemistry is expected to vary accordingly.
Two methods for achieving such an optimum substrate position have been investigated. A logical choice is to place the substrates downstream from the plasma. O'Kane and Rich found that the polymer films deposited downstream were more linear in structure (i.e., were less crosslinked) than those deposited within the plasma. D. F. O'Kane and D. W. Rice, J. Macromol. Sci., Chem., A10, 567-577 (1976). Deposition rate also decreases downstream from the plasma, however. Pender, et al., used a slightly more complex arrangement to place substrates in the vicinity of the plasma. M. R. Pender, M. Shen, A. T. Bell, and M. Millard, in ASC Symposium Series 108: Plasma Polymerization, Vol. 108, M. Shen and A. T. Bell, Eds., American Chemical Society, Washington D. C., 1979, pp. 147-160; M. Pender, M. Shen, A. T. Bell, and M. Millard, Polym. Prepr., Am. Chem. Soc., Div. Polym. Chem., 19, 516-521 (1978). They constructed a Faraday cage in which the substrates to be coated could be placed. This reduced the bombardment by highly energetic electrons and presumably only the reactive species which diffused through the cage were able to come in contact with the substrate. The deposition of several fluorocarbon precursors was investigated using XPS. These precursors all contain a double bond and have a fluorine to carbon (F/C) ratio of 2.0. The samples deposited inside the cage showed a slight increase in CF.sub.3 groups and a slight decrease in CF groups over films deposited without the cage. The authors attribute these changes to "more chain branching ending in terminal fluoromethyl groups, and less trifunctional crosslink sites or branch points." The F/C ratio was seen to increase slightly by an average of 0.2, but was still considerably less (.about.1.2) than that of the precursors (2.0).
Kay and Dilks have implied that regulation of the electron energy and density characteristics of a plasma will be important in controlling film chemistry during depositions. E. Kay and A. Dilks, J. Vac. Sci. Technol., A, 18, 1-11 (1981). This is evident because the various excitation, ionization and fragmentation processes in low temperature plasmas are known to be due primarily to electron collisions with gaseous precursors. There may be numerous ways to modify the electron energy and density distributions, several of which have been used in sputter deposition techniques (e.g., substrate biasing). The processes involved in organic deposition are considerably more complex than those in sputtering, however, and prediction of biasing effects may be difficult. Other ways of modifying electron energy and density distributions may include the use variable frequency discharges or optogalvanic mechanisms.
The following patents disclose additional information related to plasma deposition processes:
U.S. Pat. No. 3,475,307 describes a method for increasing deposition rates in glow discharge reactors. The method involves condensing a monomer vapor on a substrate and then subjecting it to a glow discharge. Instead of attempting to control the chemistry of a film as in the present invention, this patent is primarily directed to adjusting the physical mass of the films.
According to the method disclosed herein, it will be possible to control deposition rates better than with the previous technology in part because the present technique is not limited to condensation with subsequent polymerization. By decreasing the temperature of the substrate to a point above the condensation temperature of the precursor, it is possible to increase the deposition rate without subjecting the substrate to bulk condensation which is difficult to control in terms of the total amount of the material that is deposited. This is due to the difficulty in limiting condensation to a specific period of time. More elaborate cooling systems than the simple liquid cooled systems described in U.S. Pat. No. 3,475,307 are required to precisely control the amount of condensate deposited onto a surface. The present application also distinguishes between the behavior of two types of precursors, polymerizable and nonpolymerizable, used in deposition, and presents specific examples of film qualities obtained using nonpolymerizable precursors, some of which may actually show decreased deposition rates when deposited below the condensation temperature.
U.S. Pat. No. 3,068,510 is one of the first descriptions of the methods and apparatus used for glow discharge polymerization.
U.S. Pat. No. 4,212,719 describes the process where a nonvapor volume of monomer (liquid and/or solid) is exposed to an ionized gas plasma which initiates polymerization. This is essentially a bulk polymerization process, unlike that of the subject invention.
U.S. Pat. No. 4,705,612 describes a method for plasma-initiated polymerization of organic compounds. The method for plasma-initiated polymerization consists of exposing a vapor phase containing monomer vapor to a plasma and introducing a radical polymerization initiator during the exposure.
In spite of the teachings of the art, there remains an inability to adequately control the chemistry of thin films produced in a plasma deposition process.